The present invention is directed to magnetic record members used for the storage of information. This information is translated into a code, the elements of which are the direction of the magnetization of discrete areas of a magnetic medium. The information is retrieved by readout devices such as magnetic heads, which sense the direction of the magnetization retained in any of these discrete areas. The magnetic record members involved may be those such as tape, disk, drum or other forms which support the magnetic recording medium.
In order to store large quantities of information, the areas storing each element of information must be small. The distinction of the magnetization of one small area of the magnetic medium from that of contiguous areas generally requires that the magnetic sensing device be placed close to the desired element. This necessitates a precisely defined surface geometry. A high retrieval rate demands a high relative velocity between the medium and the read/write head, and this in turn requires precise surface geometry. The greater the quantity of magnetic information that can be readily stored for distinctive output, and the greater the area useful for the storage of the magnetic information, the better the economy and usefulness of the magnetic recording medium. To store the maximum quantity of magnetic information, the magnetic properties of the recording medium must have critical magnetic and geometric characteristics. These properties must be retained while subject to friction in atmospheres which may contain constituents which corrode metals.
The new magnetic coatings described herein do not require alumina particles to provide coating durability, as do some magnetic coatings. A goal of the present invention is to achieve a particulate magnetic coating which (1) is readily applied to a substrate, (2) has a high pigment volume concentration (PVC) (which is a measure of the amount of magnetic pigment or particles in the coating), (3) provides a high orientation ratio (OR) (this is a measure of the number of magnetic particles in the coating which are magnetically aligned in the desired direction), and (4) has good mechanical properties including durability.
The present coatings include polyurethane and polyacrylates as important elements. The use of polyurethane binders is well known in the art, as exemplified by U.S. Pat. Nos. 3,926,826, 3,929,659, 4,058,646, 4,074,012 and 4,152,485. A copending application assigned to the same assignee as the present application Ser. No. 304,445, filed Sept. 21, 1981 (R09-80-008), disclosed a magnetic recording composition including blocked isocyanates and allyl alcohol. However, none of these prior art coatings are believed to be as attractive as those of the present invention.
The prior art one-component polyurethane coatings have limited storage stability and little cross-linking sites, thus failing to provide good mechanical durability for extremely thin coatings in the range of 5 microinches. Though prior art two-component polyurethane coatings may have a high degree of cross-linking to give good mechanical properties, they are difficult to apply. Other one-component polyurethane magnetic coatings contain polyester diols or polyether diols which have the shortcoming of being easily hydrolyzed under a humid environment. This low hydrolytic stability causes chemical bond breakage which induces poor film-formation. The magnetic coatings are therefore broken up which causes corrosion, contamination and defects on the recording disks. The commonly used epoxy, epoxy-phenolic binder system has the disadvantages of intrinsic gel particles and the requirement for a high curing schedule. The first problem gives rise to defects and, consequently, poor magnetic performance, while the second problem means the use of more energy for coating cure.